The present invention relates generally to a process for removing thermal barrier coatings from metal components and more particularly to a method for removing a thermal barrier ceramic coating from the cooling holes of a gas turbine engine component, such as a combustor chamber liner.
Gas turbine engines (aerospace and industrial) are designed such that their nickel and cobalt based superalloy components operate at temperatures very close to their melting points. Thermal barrier coatings (TBC) perform the important function of insulating components operating at elevated temperatures. Typical turbine components are combustion chamber (see combustion chamber 10 in FIG. 1), ducts, discharge nozzles, turbine blades and nozzle guide vanes. TBCs are characterized by their very low thermal conductivity, the coating bearing a large temperature gradient when exposed to heat flow.
The most commonly applied TBC material is yttria stabilized zirconia (YSZ), which exhibits resistance to thermal shock and thermal fatigue up to 1150 degrees C. Typically the ceramic layer can be deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS) or a physical vapor deposition (PVD) process, such as electron beam physical vapor deposition (EBPVD). It is common practice to pre-coat the substrate material with a bond coat. The bond coat accommodates residual stresses that might otherwise develop in the coating system, caused by the metallic substrate and the ceramic TBC having different coefficients of thermal expansion, as well as providing oxidation and corrosion resistance. Typical bond coats include, but are not limited to, MCrAlY, wherein M is Ni, Co, Fe or mixtures thereof, or a diffusion aluminide or platinum aluminide coating.
The desire to increase the efficiency of gas turbine engines has led to an increase in the temperature in the combustion chamber and the hot section of the turbine engine. In order to compensate for the additional temperature, effusion hole cooling is often used where there is a significant heat load. Effusion hole cooling of an engine component, such as a combustion chamber 10 as shown in FIG. 1, is accomplished by laser drilling small diameter (0.010 to 0.060 inch diameter) cooling holes 11 at specific angles and patterns that deliver the required cooling air to the engine component. Effusion hole cooling systems are typically used in conjunction with TBC coatings on engine components in order to achieve maximum benefit resulting from their ability to sustain high thermal gradients. Lowering the temperature of the metal substrate prolongs the life of the engine component. In addition, these cooling hole and TBC systems reduce the thermal gradients in the metal substrate thereby reducing the driving force for thermal fatigue. The benefit of these systems is realized in greater component durability, higher gas temperature, performance and improved efficiency.
Laser drilling (e.g. Nd: YAG laser) is used to drill and manufacture cooling holes in gas turbine engine hot section components. These parts are preferably protected by thermal barrier coatings (TBC). The laser drilling process can manufacture the cooling holes by drilling through the component's metallic substrate and the TBC at the same time; however, laser induced damage occurs during this manufacturing process. Microstructural damage is generated at the TBC interface with the metallic bond coat and metallic substrate which results in TBC debonding and subsequent ceramic insulation coating loss (spallation) which is detrimental to the metallic substrate due to the high heat loads which adversely affects part durability and service life.
Various techniques have been developed to remove thermal barrier coatings from components during manufacture and repair, including air-cooled components. U.S. Pat. No. 6,004,620, EP 1340587 A2 and U.S. Pat. No. 6,620,457 B2 disclose a waterjet system with or without particulate media (abrasive or non-abrasive) utilizing a liquid-containing jet which operates at high fluid pressures ranging from 5000 pounds per square inch to 50,000 pounds per square inch in order to remove thermal barrier coating deposits. The waterjet process creates “minimal” wear and erosion of the underlying substrate after only a single cycle at 5000 pounds per square inch pressure. Additional cycles and/or increased pressures provides wear and erosion beyond what is considered minimal.